Process of producing white tanned leather by precipitating in situ zinc sulfide and sulfhydrate compounds and product produced therefrom



United States Patent O PROCESS OF PRODUCING WHITE TANNED LEATHER BY PRECIPITATING IN SITU ZINC SULFIDE AND SULFHYDRATE COMPOUNDS AND PRODUCT PRODUCED THEREFROM Harlan A. Depew and George R. Waitkins, Kirkwood, Mm, assignors to American Zinc, Lead & Smelting Company, St. Louis, Mo., a corporation of Maine No Drawing. Application May 13, 1954,

' Serial No. 429,671

2 Claims. (Cl. 8-94.25)

This invention relates to processes for the manufacture of leather tanned with Zinc compounds and to the resulting white leather products.

More specifically, the present invention relates to new and improved processes for tanning leathers using zincsulfur compositions and tothe resulting leather products.

One of the most satisfactory commercial processes for producing white leather is the alum process in which basic aluminum compounds are deposited in the hide and a fair white product is obtained. This product leaches readily in water and is not stable at elevated temperatures.

Other tanning processes for obtaining white and near white leather are known including the zirconium process and the syn-tan surface bleach of a chrome tan.

United States Patents Nos. 504,012 and 504,014 disclose partly tanning a hide with chrome and then adding zinc sulfate to the chrome tan liquor. Suflicient KzS is then added to react with a part of the zinc salt. Manganese sulfate may also be added. The unreacted ZnSO4 and precipitated ZnS and MnS do not diffuse into the partly tanned leather with the chromium salts that effected the tanning. Zinc sulfide may be added to a chrome tan leather for its value as a filler.

When zinc sulfide is precipitated in a colloidal form and a properly prepared hide is immersed in the slurry, the colloidal zinc sulfide does not penetrate the hide, but on boiling, any zinc sulfide particles would migrate out and the chemical stability and other leatherlike properties of the hide are unsatisfactory.

Improved processes for tanning leather and the resulting products are described in the co-pending application of Harlan A. Depew which discloses the use of zinc hydroxide and related compoundsfor tanning leather. Chemical combination of a part of the zinc with the hide substance holds the zinc in place during leaching while some of the uncombined zinc slowly leaches. This process not only gives a better color and better stability as compared to alum tanning, but the reduced rate of loss of whiteness on leaching is important.

The novel processes of the present invention include the steps of precipitating a zinc sulfide or sulfhydrate in situ in .the leather, and we have found that the resulting novel leather products of our invention have recognized leather qualities when heated in boiling water. Generally speaking, these leather products will test better than leather tanned with vegetable tannin but not as well as leather tanned with chrome. However, these processes produce an excellent tanned normally white product, i. e., a product whichis white unless intentionally colored during the tanning process.

It is accordingly an object of the present invention to provide novel process for the tanning of leather using zinc-sulphur compositions.

Another object is to provide novel leather products containing zinc-sulphur compositions.

Another object is to provide such processes in which zinc-sulphur compositions are precipitated in situ in the hide to tan it.

Another object is to provide such leather products containing precipitated zinc-sulphur compositions.

Another object is to provide such processes in which the hide is first treated with a soluble sulfide or hydrosulfide and then with a zinc salt to form a basic zinc sulfide in the hide.

Another object is to provide such leather containing basic zinc sulfides.

Another object is to provide such processes in which the hide may be subsequently supplementally tanned with known tanning materials.

Other and further objects of the present invention will appear from the following description thereof.

The difficulties of the prior art are overcome if, in accordance with the present invention a hide is first treated with a soluble sulfide or 'hydrosulfide such as KzS, NaSH, Ba(SH)z or Ca(SH)2. The sulfide is apparently taken up by the hide giving it a greenish color and an alkaline reaction. NazS, (NH4)2S and other alkali sulfides may be used, but they are somewhat harsher alkalis than may be preferred. The color of the hide treated with NazS or (NH4)2S is darker and yellower.

The hide, after being impregnated with the sulfide solution, is then immersed in a solution of a zinc salt such as ZnClz, ZnSO4, or'an ammoniacal solution of these compounds or their basic salts. When an acid zinc salt with a pH of about 4 /z-5 /2 is used, the hide that originally had a pH of 9-10 drops in pH during treatment to around 3-4. This indicates that a basic zinc sulfide is formed in the hide as follows;

Zn(OH)z dissolves at a pH of 4-5, but the basic zinc sulfide above does not dissolve rapidly. This basic zinc sulfide makes an even more stable leather than does Zn(SH)2.

When zinc sulfide is referred to hereinafter and in the claims it is to be understood that ZnS, Zn(SH)2 and basic zinc sulfides are meant. V

It is believed that the zinc is precipitated in the hide as a basic zince sulfide and that the basicity becomes greater from the outside of the hide toward the center.

A piece of leather tanned by our process was sectioned into five layers and it was found after boiling that the skin layer of the hide lost only a small amount of zinc, the fatty layer, a little more, and the center layer, the most. However, even in the center layer the residual zinc was retained in a considerable quantity. The color in the outer layers was unaffected, but it was largely lost in the inner layers.

As examples of our processes the following solution were prepared:

A piece of bated hide was placed in a ball mill containing a few pebbles and solution A and was milled for 4 hours. The hide was then removed, washed lightly, and with some pebbles put into another mill containing solution B where it was milled for three hours, left in theliquid over night, and then remilled for 2 hours. After a lightwash the hide was put into a mill with solution C and milled for 1 /2 hours. The leather was then washed and dried. The dried leather was split into five layers of uneven thickness and tested and analyzed.

I Original dimensions 1 x 2.

It is evident that although the skin side of the hide was tanned best, the product was fairly uniform throughout.

Analyses of the layers showing zinc extraction during boiling were:

Amount Zinc Percent; Layers inc Content Leached, In

Percent Residue,

Percent 3. 9 l. 2. 9 4. 3 l. 0 3. 3 3. 3 l. 4 1. 9 2. 6 0. 4 2. 2 6. 6 0. 6. 1

Regardless of the length of leaching, the color in layers 1, 2, 3, and 5 remained good, and the tanning throughout.

the hide was good.

In this example, the timing and concentrations were such that the hide had a higher zinc content in layer 5 than in layer 1. This may have been due to different protein compositions of the two layers.

It is believed that when a hide is treated with Na(SH), for example, the SH group is adsorbed and increasingly alkaline liquor moves through the hide. When the hide is then treated with ZnSO4, Zn(OH)2 and Zn(SH)z are precipitated and complexes are formed of the protein and the zinc compounds that cross link the protein molecules and produce the tanning action.

The bestresults are obtained by ending on the acid side with all the free sulfide reacted by zinc. This is desirable in order to increase the SH to OH ratio in the zinc deposit. This prevents discoloration apparently due to a local excess of unreacted alkali sulfide.

During the zinc salt treatment of the hide and the development of acidity, some HzS is formed and ZnS is precipitated. This freshly precipitated colloidal ZnS is adsorbed in the hide and held much more strongly than any colloidal ZnS that may have migrated into the hide from outside.

Example 11 Ten calf skins weighing about 400 lbs. were placed in a rotating drum with the following solution:

NaSH lbs 19 Salt (NaCl( .lbs 19 Water gallons 39 After two hours drumming, the hides had a uniform light green color. The hides were drummed for another two hours when penetration to the center seemed complete.

The solution was drained and the hides were washed with water for 15 minutes.

The following solution was then added to the drum and the hides were drummed one hour:

ZnSO4.H2O lbs 19 NaCl lbs 19 Water gallons 38 The hides were then allowed to stand overnight in the zinc liquor following which they were drummed for 3% hours.

At this stage, the hides were uniformly white and the surface was easily indented by the fingers and the hide structure appeared to be fibrous throughout.

The hides were then washed with water and oiled. The hides took oil readily, showing an open structure. The finished leather analyzed 3.7% zinc.

The leather was cut into thin strips and tested for tensile strength with a good resultant value of 6800 lbs./ sq. in.

The leather ages well, attributable to the basic ZnS tan.

Example 111 ll.l parts of anhydrous CaClz were dissolved in parts of water. To this was added a solution of 18.4 parts of NaSH.2H2O, also in 85 parts water, to obtain a pale, greenish-yellow solution. A 4 x 8" piece of bated calfskin was milled in this solution for 4 hours. The hide was drained and then returned to the mill with a solution consisting of 22 parts ZnSO4.H2O, 10 parts salt, and parts water, and was milled for two hours, soaked in the milling solution overnight, and again milled for 5 hours. The tanned hide was removed, rinsed thoroughly with water, and milled for 2% hours in an emulsion consisting of 15 parts anionic sulfonated oil, 10 parts salt, and 170 parts water. The oiled hide was washed lightly with water and air-dried by hanging at normal room temperature to obtain a very pliable, white leather having good properties. This leather analyzed 6.4% zinc and showed a shrink temperature of 68-70.

Example IV 20.3 parts of MgCl2.6H2O were dissolved in 85 parts of water. To this was added a solution of 18.4 parts of NaSHZHzO, also in 85 parts water, to obtain a pale, greenish-yellow solution. A 3" x 8" piece of bated calfskin was milled in this solution for 4 hours. The hide was drained and returned to the mill with a solution consisting of 22 parts ZnSO4.H2O, 10 parts salt, and 170 parts water, and was milled 2 /2 hours, soaked in the milling solution overnight, and again milled for 4 hours. The tanned hide was removed, rinsed thoroughly with water, and milled for 3 hours in an emulsion consisting of 15 parts anionic sulfonated oil, 10 parts salt, and 170 parts water. The oiled hide was washed lightly and air dried by hanging at normal room temperature to obtain a pliable, white leather having good properties. This leather analyzed 4.7% Zn and showed a shrink temperature of 6468.

Example V A 4" x 8" piece of NH3 treated, bated hide was washed with water to remove any NHs on its surface. The hide was milled for 4 hours in a solution of 10 parts NaSH, 20 parts NaCl, and 170 parts water. The hide was then washed with water and milled for 3 hours in a solution of 22 parts ZnSO4.H2O, 10 parts NaCl, and 170 parts water. It was soaked overnight in the milling solution, then milled for 4 additional hours. It was then well tanned. A solution of 5 parts CI2(SO4)3 in 25 parts of water was prepared and added to the ZnSO-r milling solution containing the hide. This solution was milled for one hour. The well impregnated hide Was then washed with water and milled for 2 hours in a solution of 15 parts anionic sulfonated oil, 10 parts salt, and 170 parts water. The oiled hide was Washed lightly with water and hung to air dry at normal room temperature. The dried leather was very pliable, a slightly greenish off-white, and had excellent shrink resisting qualities.

Example VI A 4" x 8" piece of NH3 treated, bated hide was washed with water to remove any NHs on its surface. The hide was milled for 4 hours in a solution of 10 parts NaSH, 20

parts NaCl, and 170 parts water. The hide was then washed with water and milled for 3 hours in a solution of 22 parts ZnSO4.H2O, parts NaCl, and 170 parts water. It was soaked overnight in the milling solution, then milled for 4 additional hours. 5 parts of 40% HCHO solution were added to the mill and milling continued for 2 hours. The hide was washed with water and milled 2 hours in a mixture of parts anionic sulfonated oil, 10 parts NaCl, and 170 parts water. The hide was washed with water and hung to air-dry at normal room temperature. The dried leather was quite pliable, very white, with good stability.

Although (NH4)2S gives good results and makes a high grade product in these processes, HzS is evolved requiring properly ventilated equipment and plant. Accordingly, NaSH is normally preferred for commercial operation.

If additional tanning as with formaldehyde or chromium salts is desired, it can be accomplished at the end of tanning during the oiling step. If formaldehyde is added with NaSl-I, a bad garlic odor persists after the leather is finished.

It should now be apparent that the present invention in every way satisfies the objects described above. Further, changes in or modifications to the above described illustrative embodiments of this invention may now be suggested to those skilled in the art without departing from the present inventive concept. Reference should therefore be had to the appended claims to determine the scope of this invention.

What is claimed is:

1. A process for tanning hides consisting of the essential steps of impregnating the hides with a water soluble sulfide selected from the group consisting of potassium, sodium, barium, calcium, and ammonium sulfides, and then impregnating the hides with a zinc salt selected from the group consisting of zinc chloride and zinc sulfate, there being sufficient quantities of water soluble sulfide and zinc salt employed to precipitate in the hides sufiicient amounts of a zinc sulfide selected from the group consisting of zinc sulfide, zinc sulfhydrate, and basic zinc sulfides to tan the hides.

2. A pliable leather of fibrous structure substantially tanned and normally white throughout and substantially free from leaching of the tanning agent in water obtained by the process defined in claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 281,411 Smallridge July 17, 1883 504,012 Zahn Aug. 29, 1893 504,014 Zahn Aug. 29, 1893 915,638 Trenckmann Mar. 16, 1909 

1. A PROCESS FOR TANNING HIDES CONSISTING OF THE ESSENTIAL STEPS OF IMPREGNATING THE HIDES WITH A WATER SOLUBLE SULFIDE SELECTED FROM THE GROUP CONSISTING OF POTASSIUM, SODIUM, BARIUM, CALCIUM, AND AMMONIUM SULFIDES, AND THEN IMPREGNATING THE HIDES WITH A ZINC SALT SELECTED FROM THE GROUP CONSISTING OF ZINC CHLORIDE AND ZINC SULFATE, THERE BEING SUFFICIENT QUANTITIES OF WATER SOLUBLE SULFIDE AND ZINC SALT EMPLOYED TO PRECIPITATE IN THE HIDES SUFFICIENT AMOUNTS OF A ZINC SULFIDE SELECTED FROM THE GROUP CONSISTING OF ZINC SULFIDE, ZINC SULFHYDRATE, AND BASIC ZINC SULFIDES TO TAN THE HIDES. 